Although Ca.sup.2+ is a well-established intracellular messenger, there are still many unanswered questions concerning the kinetics and spatial localization of its effects. As a means of supplying the data needed to answer some of these questions, chelators whose affinity for calcium is decreased by illunination, such as those disclosed in U.S. Pat. No. 4,689,432, are now being used to induce fluctuations in free calcium levels similar to those which occur in vivo. Also see, Gurney and Lester (1987).
Although the photolabile chelator compounds disclosed in U.S. Pat. No. 4,689,432 are useful for examining some processes which involve calcium, unfortunately the calcium 2+ affinity of some of these compounds is not strong enough for studying others. For example, with chelators such as nitr-2, a compound disclosed in U.S. Pat. No. 4,689,432, at values of cytosolic free calcium 2+ typical for resting cells, only about 1/2 or less of the uphotolyzed chelator molecules bear calcium 2+ ions. Obviously the less calcium 2+ ions that are bound over to the chelator, the less that can be released by photolysis. In addition, those molecules which escape photolysis during the light flash tend to buffer the free calcium rise generated by those molecules which do photolyse.
The present invention discloses a new class photolabile calcium chelator compounds. Unlike the calcium chelators disclosed in U.S. Ser. No. 648,536, in which the two halves of the chelator are linked by a simple 1,2-ethanediyl (--CH.sub.2 CH.sub.2 --) moiety, the compounds of the present invention modify the stereochemical conformation of this linkage by adding bulky substituents or incorporating the linkage into a carbocyclic or heterocyclic ring. Such stereochemical modifications can decrease the Ca.sup.2+ affinity 75-fold or increase its 4-fold compared to prior art compounds. The new compounds also release Ca.sup.2+ more rapidly after photolysis than do compounds of the prior art such as nitr-2. These differences make the compounds of the present invention especially useful for studying the effects of Ca.sup.2+ on such phenomena as ion-channel gating and muscle contraction.